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Register a new userStructured beams invariant to coherent diffusion
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Bessel beams are renowned members of a wide family of non-diffracting (propagation-invariant) fields. We report on experiments showing that non-diffracting fields are also immune to diffusion. We map the phase and magnitude of structured laser fields onto the spatial coherence between two internal states of warm atoms undergoing diffusion. We measure the field after a controllable, effective, diffusion time by continuously generating light from the spatial coherence. The coherent diffusion of Bessel-Gaussian fields and more intricate, non-diffracting fields is quantitatively analyzed and directly compared to that of diffracting fields. To elucidate the origin of diffusion invariance, we show results for non-diffracting fields whose phase pattern we flatten.
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Partially coherent light is abundant in many physical systems, and its propagation properties are well understood. Here we extend current theory of propagation of partially coherent light beams to the field of coherent diffusion. Based on a unique four-wave mixing scheme of electro-magnetically induced transparency, an optical speckle pattern is coupled to diffusing atoms in a warm vapor. The spatial coherence propagation properties of light speckles is studied experimentally under diffusion, and is compared to the familiar spatial coherence of speckles under diffraction. An analytic model explaining the results is presented, based on a diffusion analogue of the famous Van Cittert-Zernike theorem.
The multiple scattering of photons in a hot, resonant, atomic vapor is investigated and shown to exhibit a Levy Flight-like behavior. Monte Carlo simulations give insights into the frequency redistribution process that originates the long steps characteristic of this class of random walk phenomena.
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We investigate the transient coherent transmission of light through an optically thick cold stron-tium gas. We observe a coherent superflash just after an abrupt probe extinction, with peak intensity more than three times the incident one. We show that this coherent superflash is a direct signature of the cooperative forward emission of the atoms. By engineering fast transient phenomena on the incident field, we give a clear and simple picture of the physical mechanisms at play.
We here report coherent reflection of thermal He atom beams from various microscopically rough surfaces at grazing incidence. For a sufficiently small normal component $k_z$ of the incident wave-vector of the atom the reflection probability is found to be a function of $k_z$ only. This behavior is explained by quantum-reflection at the attractive branch of the Casimir-van der Waals interaction potential. For larger values of $k_z$ the overall reflection probability decreases rapidly and is found to also depend on the parallel component $k_x$ of the wave-vector. The material specific $k_x$ dependence for this classical reflection at the repulsive branch of the potential is explained qualitatively in terms of the averaging-out of the surface roughness under grazing incidence conditions.
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